1. Field of the Invention
The present invention relates to an optically controlled optical-path-switching-type optical signal transmission apparatus and to a method of switching optical paths for optical signals, that are used in the fields of optical communications and of optical information processing.
2. Description of the Related Art
In order to cope with the explosive increase of network traffic along with the expansion of the Internet and corporate and domestic intranets, an optical path switching apparatus not involving any electric signal (optical switch), i.e., a light-to-light direct switch is being sought. As an optical fiber, an optical waveguide, or an apparatus for or a method of switching courses for light beams propagating in space to travel, i.e., optical paths, schemes are known such as, for example, a space division scheme in which s optical paths are switched in an optical waveguide or between optical waveguides, a wavelength division multiplexing scheme in which a multiplexed light beam having a plurality of wavelengths is switched by dividing the light beam for optical paths according to the wavelength, a time division multiplexing scheme in which optical paths of light beams that are time-division-multiplexed at an constant time interval are switched, and a free space scheme in which spatially optical paths of light beams propagating through space are divided and coupled using a mirror or a shutter. Each of these schemes can be multiplexed, or a plurality of schemes can be used in combination.
Proposed space-division-type optical switches include those that utilize a directional coupler, those that create a copy of an optical signal using an optical dropper and switch a light beam between ON and OFF using a gate device, those that transmit or reflect a light beam propagating a waveguide by varying the refractive index of the waveguide at a crossing portion of an intersection or a Y-shaped branching point, and others. However, all of these remain in the stage of research and development. Apparatuses employing a thermo-optical effect created by using an electric heater to vary the refractive index of a waveguide of a Mach-Zehnder-interferometer-type optical waveguide switch are approaching practical application, but such apparatuses re disadvantageous in that this type of apparatus has a low response speed, of approximately 1 millisecond, and also requires an electric signal to operate the optical switch.
Meanwhile, available free-space-type optical switches include a micro-electro mechanical system (abbreviated to MEMS), an exciton absorption reflection switch (abbreviated to EARS), a multi-stage-beam-deflector-type optical switch, a hologram-type switch, a liquid crystal switch, and others. However, these switches cannot be said to be sufficiently developed for practical use because they have assignments such as that they have mechanically movable portions; they are dependent on polarized electromagnetic radiation, and other factors.
On the other hand, there is active study of total-light-type optical devices or optical control methods that modulate the intensity or the frequency of a light beam directly by utilizing variation of the transmittance or the refractive index caused when an optical device is irradiated with light. The inventors of the invention described in the present application are continuing an ongoing study of an optical control method aimed at development of a new information processing technique with a total-light-type optical device, etc. using an organic nanoparticle thermo-optical lens forming device formed by dispersing organic pigment aggregate in a polymer matrix (see Takashi Hiraga, Norio Tanaka, Kikuko Hayamizu and Tetsuo Moriya, “Formation, Structure Evaluation and Photo-Material Property of Associated/Aggregated Pigment”, Journal of Electronic Technology General Institute, Electronic Technology General Institute, Agency of Industrial Science and Technology, Ministry of International Trade and Industry, Vol. 59, No. 2, pp. 29-49 (1994)). Currently, a device employing a scheme that modulates a signal light beam (780 nm) by a control light beam (633 nm), having a characteristic that the control light beam and the signal light beam are coaxial and have incidence of the same focal point, and based on an operational principle that the signal light beam is refracted by a thermal lens formed transiently by absorption of the control light beam, is being developed and a high-speed response of approximately 20 nanoseconds has been achieved. Japanese Patent Application Laid-Open Publications Nos. 1996-286220, 1996-320535, 1996-320536, 1997-329816, 1998-90733, 1998-90734 and 1998-148852 disclose an optical control method of carrying out intensity modulation and/or light flux density modulation of a signal light beam transmitted through an optical device by reversibly varying the transmittance and/or the refractive index of the signal light beam in a different wavelength band from that of the control light beam by irradiating the optical device comprising optically responsive composition, with the control light beam, wherein the control light beam and the signal light beam are converged and irradiated on the optical device, and the optical paths of the control light beam and the signal light beam are adjusted such that an area having the highest photon density in the vicinity of a focus (beam waist) of each of the control light beam and the signal light beam are overlapped on each other in the optical device. Furthermore, Japanese Patent Application Laid-Open Publication No. 1998-148853 discloses an optical control method of carrying out intensity modulation and/or light flux density modulation of a signal light beam transmitting a thermal lens by reversibly forming the thermal lens based on the distribution of density variation caused by a temperature increase generated in an area of the photo-responsive composition, that has absorbed the control light beam and the surrounding area thereof, wherein a control light beam and the signal light beam having a wavelength different from each other are irradiated on an optical device comprising photo-responsive composition, the wavelength of the control light beam is selected from a wavelength band that the photo-responsive composition absorbs. Yet further, in Japanese Patent Application Laid-Open Publication No. 1998-148853, it is described that a pigment/resin film or a pigment solution film is, for example, used as the optical device and a response time of the signal light beam against the irradiated control light beam for the case where the control light beam has a power of 2 to 25 mW is shorter than 2 μsec.
Here, the thermal lens effect is a refractive effect in which molecules, etc. that have absorbed light in the central area of light absorption convert the light into heat, a temperature distribution is created by propagation of this heat to the surrounding area, and, as a result, the refractive index of an optical transmitting matter is varied spherically from the center of the light absorption to the outer region to create a distribution for the refractive index which is lower at the center of the light absorption and higher continuing outward, with functions similar to those of a convex lens. The thermal lens effect has long been utilized in the field of spectral analysis, and an ultra high sensitivity spectral analysis can be carried out that can detect the light absorption of even a single molecule (see Kitao Fujiwara, Keiichiro Fuwa and Takayosi Kobayasi, “A Laser-Induced Thermal Lens Effect and Its Application to Calorimetry”, Chemistry, Kagaku-Dojin, Vol. 36, No. 6, pp. 432-438 (1981); Takehiko kitamori and Tsuguro Sawada, “Photo-Thermo Conversion Spectral Analysis Method”, Bunseki, Japanese Society of Analytical Chemistry, March, 1994, pp. 178-187).
Moreover, Japanese Patent Application Laid-Open Publication No. 1985-14221 discloses, as a method of deflecting an optical path using variation of refractive index caused by the thermal lens effect or heat, a method of deflecting a light beam by creating a distribution of refractive index in a medium by providing heat using a heating resistor.
However, because, in all of the above methods, heat is produced using a heating resistor and a medium is heated using conduction, these methods have an intrinsic problem of diffusion of heat. That is, because of the diffusion of heat, a fine thermal gradient cannot be provided over a large area and a desired distribution of the refractive index cannot not be easily or reliably obtained. Furthermore, in actual practice, the fine processing of a heating resistor is limited, even when a photolithography technique used for semiconductor integrated circuits is employed, such that it is not possible to prevent the size of the device from increasing. When the size of the device increases, the optical system becomes larger and more complicated. Furthermore, because heat is produced using a heating resistor and the medium is heated by conduction of the heat, this invention has intrinsic disadvantages such as that the response is slow and the frequency for varying the refractive index cannot be increased.
Moreover, Japanese Patent Application Laid-Open Publication No. 1999-194373 discloses a deflecting device using an optical device, comprising at least the optical device comprising an photo-sensitive composition and intensity distribution adjusting means for irradiating the optical device with light in a wedge-shaped optical intensity distribution, wherein a distribution of refractive index is formed in the optical device by a control light beam and deflection of a signal light beam having a wavelength different from that of the control light beam is carried out by the distribution of the refractive index. Although this scheme is excellent in terms of controlling light using light, this scheme is constrained in that the angle of deflection must be within 30 degrees and, therefore, is problematic in that directions for switching optical paths cannot be freely set.
Then, the inventors filed a patent application describing an optical path switching method as described below which provides an optical path switching apparatus and an optical path switching method having no polarized-electromagnetic-wave dependence, for which angles and directions for switching optical paths can be set freely, with which optical intensity attenuation of a signal light beam is small, and which can be used in multiple connection. In this method of switching optical paths, which is disclosed in Japanese Patent Application Laid-Open Publication No. 2004-109892, a control light beam having a wavelength selected from a wavelength band that a light absorbing layer film absorbs and a signal light beam having a wavelength selected from a wavelength band that the light absorbing layer film does not absorb are respectively converged and irradiated on the light absorbing layer film in a thermal lens forming device containing at least the light absorbing layer film; arrangement is adjusted such that at least the control light beam is focused within the light absorbing layer film; and a thermal lens based on a distribution of the refractive index created reversibly caused by a temperature increase produced in an area of the light absorbing layer film that has absorbed the control light beam and the area surrounding the area is used. Thereby, a state where the converged signal light beam exits from the thermal lens forming device at an ordinary divergence angle in the case when the control light beam is not irradiated and no thermal lens is formed, and another state where the converged signal light beam exits from the thermal lens forming device at a divergence angle larger than the ordinary divergence angle in the case when the control light beam is irradiated and a thermal lens is formed are realized in response to the presence or absence of the irradiation of the control light beam; in the case where the control light beam is not irradiated and no thermal lens is formed, the signal light beam existing from the thermal lens forming device at the ordinary divergence angle is as is, or after changing the ordinary divergence angle using a light-receiving lens, directed to travel straight through a hole of a mirror provided with a hole to pass the signal light beam; but, when the control light beam is irradiated and a thermal lens is formed, the signal light beam exiting diverging from the thermal lens forming device at a divergence angle larger than the ordinary divergence angle is as is, or after changing the divergence angle of the divergence using a light-receiving lens, reflected using the mirror provided with the hole.